1. Field of the Invention
The present invention relates to systematic methods and apparatus for stressing dispersions and emulsions in order to accelerate the onset of particle agglomeration or droplet coalescence and thereby assess their stability and evaluate their quality and performance.
2. Description of the Related Art
There are many products and intermediate process materials in widespread commercial use that depend on fine particles dispersed or suspended in a liquid, frequently water, thus classified as 2-phase systems. These particles may be solid, as in the case of aqueous polymer suspensions. Alternatively, the “particles” may consist of droplets of a liquid normally immiscible in the suspending fluid—e.g. oil droplets, in the case of oil-in-water emulsions, or water droplets (usually containing water-soluble species), in the case of water-in-oil emulsions. As well, more complex dispersions exist as multi-phase systems, forming, for example, water-in-oil-in-water and oil-in-water-in-oil emulsions.
The particles or droplets comprising the “dispersed phase” usually possess a variety of sizes, often ranging from 0.1 micrometer (micron, or μm), or smaller, to 1 μm, or larger. The distribution of particle or droplet sizes for a particular product depends on the chemical composition of the dispersion or emulsion in question (including the dispersant(s) used in conjunction with the particles or droplets) and the physical mechanism(s) used to achieve the final product. Examples of the latter include homogenization, in the case of an oil-in-water emulsion, and emulsion polymerization, in the case of a polymer suspension. Whether the dispersed phase consists of solid particles or liquid droplets, it is convenient to refer to the distribution of particle or droplet sizes as the “particle” size distribution, or PSD, by which usage no loss of generality is implied or intended. Use of the word “particles” herein is intended to include both solid particles and liquid droplets. Likewise, it is also convenient to refer to the products as “dispersions” by which no loss of generality is implied or intended. Therefore, the use of the word “dispersion” herein is intended to include both emulsions and suspensions.
There are numerous examples of widely used products based on dispersions, suspensions or emulsions. The following list is intended to be representative of the wide range of existing applications, but it is by no means complete:    a) inorganic (e.g. silica, alumina and cerium oxide) colloidal suspensions, used for chemical mechanical planarization (CMP) processing of silicon wafers during fabrication of semiconductor devices;    b) aqueous polymer suspensions, used for paints, coatings, adhesives and sealants;    c) edible oil-in-water emulsions (containing flavor and color), used for beverages and food products, such as sauces, dressings and dietary supplements;    d) silicone-based emulsions, used for hair cleaning (shampoos) and conditioning, hand lotions and surgical scrubs, as well as sealants, flexible potting compounds and medical implants;    e) wax- and/or clay-containing aqueous emulsions, used in cosmetic preparations;    f) soybean-, safflower-, olive-, medium-chain triglyceride- and/or fish-oil based oil-in-water emulsions, used for intravenous drug delivery (e.g., anesthesia) and parenteral nutrition;    g) pigment-based suspensions, used for both conventional and ink-jet printing;    h) silane-based oil-in-water emulsions, used for water repellence applications;    i) inorganic (e.g. titanium-oxide) colloidal suspensions, used for pigmentation and sunscreens;    j) homogenized whole-milk (or fat-reduced) dispersions;    k) water-in-oil emulsions and microemulsions, used for lubricants and fuels;    l) oil-in-water emulsions, used for ultrasound contrast imaging;    m) asphalt-based oil-in-water emulsions, used for road maintenance.
The effectiveness of these and other dispersion- or emulsion-based products depends critically on their stability, both on the shelf and during use. In the context of the present invention, the term “stability” refers to the tendency of the particles comprising the dispersed phase to remain separated, without significant agglomeration or coalescence, over an extended (ideally, indefinite) period of time. The stability of a given dispersion or emulsion is typically a function not only of its chemical composition, but also of the “details” of the manufacturing process involved. The “quality,” or performance, of these dispersion- or emulsion-based final products and intermediate process materials is often related to their stability—i.e. to the underlying forces that can drive them toward destabilization. The performance of these products usually correlates strongly with the PSD of the dispersion or emulsion in question. Examples of important physical properties that are affected by the PSD include viscosity, hardness, strength, conductivity (thermal and electrical), appearance, color, hue, gloss, taste and texture.
Physicochemical stability is an important outcome for the use of any commercially available suspension or dispersion. An unstable dispersion is characterized by adverse changes in the spatial distribution of the dispersed phase, such that otherwise homogeneously distributed fine particles or droplets irreversibly agglomerate or coalesce, respectively. In the case of liquid dispersions (e.g. oil-in-water or water-in-oil emulsions), the liquid droplets coalesce, resulting in ever larger, over-size “globules,” ultimately leading to phase separation on a macroscopic scale. This process of particle agglomeration or droplet coalescence results in an increase not only in the mean particle/droplet size, but also in the “polydispersity,” or range of particle/droplet sizes, comprising the overall PSD. The most devastating effects of agglomeration or coalescence in a commercially prepared dispersion or emulsion often occur in a size range that is approximately ½ to 1 log larger than the mean particle or droplet size. This size range where significant changes occur is referred to as the large-diameter “tail” of the PSD and is usually of the greatest interest. The relatively remote population constituting the tail of the PSD for stable dispersions generally accounts for less (often, substantially less) than 1% of the overall dispersed phase, on either a number- or volume-weighted basis. Growth in the tail of the PSD, relative to “baseline” values obtained for “good” (stable, high-quality) products, can occur at any stage—i.e. during manufacturing, storage or distribution of products, or during subsequent improper, or sub-optimal, use by the industrial or private consumer.
Failures may occur in a variety of ways that make the final manufactured product unattractive, unusable or even dangerous in the hands of the consumer. There are numerous common examples of such failures, including: poor taste associated with an unstable beverage concentrate, dairy product, food dressing or sauce; poor appearance or orifice clogging by an ink, caused by large-particle agglomerates; creation of defects (e.g. scratches) on the surface of a silicon wafer during polishing by a CMP slurry, resulting in reduced yield of semiconductor devices; poor or incomplete coating of surfaces by multi-component paints or protective finishes, due to flocculation of latex particles or wax emulsion droplets, respectively; injury to humans or animals due to intravenous administration of nutrients or drugs containing excessive amounts of coalesced oil droplets or agglomerated/fused liposomes.
The economic costs of these and other failures of dispersions or emulsions at the industrial level can be considerable, especially those associated with internal loss of either intermediate materials or end products resulting from sub-optimal production techniques. These losses can occur during new product development or in the production of established products that have wide batch-to-batch variations due to variables that are poorly understood or difficult to monitor and control. Even higher losses are incurred when inferior products escape pre-market detection protocols and must subsequently be recalled in large quantities. In addition, the industrial consumer's large-scale use of dispersions or emulsions, as either raw materials or essential components of final commercial products, entails exposure to potentially large economic losses. Finally, if the product is intended for internal human use or contains extremely volatile or combustible components, the subsequent adverse, or even lethal, consequences amplifies the health and economic risks associated with product failures.
In many cases the products or materials of interest are colloidal in nature, in which the great majority of particles (or droplets) are smaller than 1 μm, with population mean diameters typically lying in the range of 0.05–0.10 μm (“micro-emulsions”), 0.10–1.0 μm (“mini-emulsions”) or 1.0–10.0 μm (“macro-emulsions”). Determination of the PSD, assuming that the measurement technique possesses adequate sensitivity and resolution, provides a valuable, and often unique, “window” on both the stability and quality (both current and prospective) of these dispersions and emulsions. Such information is potentially extremely valuable, as it can provide a quantitative, objective yardstick for judging the efficacy of a given manufacturing process, and perturbations of the latter. Ultimately, this knowledge can inform the means for optimizing the manufacturing processes used to generate the product in question.
As is evident from the list of applications above, the most widely encountered class of dispersions and emulsions are aqueous systems, in which the “continuous” phase surrounding the dispersed phase of particles or droplets consists of water. (The aqueous phase is not purely water. It invariably contains one or more species of charged, mobile ions (electrolyte) plus possibly other dissolved molecules, both charged and uncharged, together with whatever concentrations of H+ and OH−ions are required to establish the pH of the dispersion medium.) Consequently, the most widely employed physical mechanism for stabilizing aqueous dispersions and emulsions is that of charge stabilization. The first comprehensive description of this mechanism was provided by Verwey and Overbeek, in their classic monograph: Theory of Stability of Lyophobic Colloids, Elsevier Science, Amsterdam (1948). Additions to the theory were provided by Derayaguin and Landau, and the resulting “DLVO Theory” of colloid stability (due to Derayaguin, Landau, Verwey and Overbeek) has been reviewed by numerous authors, including Hiemenz, in: Principles of Colloid and Surface Chemistry, Marcel Dekker, New York (1977). This theory provides a quantitative description of the competition between the two fundamental, opposing forces that exist between neighboring particles suspended in an aqueous medium—Coulombic (electrostatic) repulsions and London-van der Waals attractions.
The electrostatic interparticle repulsive force arises from the net charge (either positive or negative) residing on the surfaces of the particles. This charge may result from moieties that are covalently bonded to the surfaces—i.e. which are an intrinsic part of the particle in question—and which lose their neutrality by virtue of the dissociation of ions into the surrounding aqueous phase. The extent to which these oppositely charged “counterions” are released into the continuous phase, thereby leaving a net charge on the surface, depends on the value(s) of the individual dissociation constant(s), or pKa(s), of the “titratable,” surface-bound moieties, relative to the pH of the surrounding aqueous phase. One example would be carboxylated polystyrene latex particles—i.e. latex “beads” containing COOH groups, covalently bound to their surfaces. If the pH of the aqueous phase is significantly greater than the pKa of the COOH groups (i.e. ≧2 pH units above the pKa value), most of the H+ions will become dissociated from the latter, leaving negatively charged COO− groups attached to the surfaces of the particles. Conversely, as the pH of the aqueous phase approaches the pKa value (i.e. pH=pKa), only 50% of the H+ ions are dissociated. At its worst, if the pH falls below the pKa value, the surface charge greatly diminishes, even approaching zero, because more H+ ions are now adsorbed onto the particles, and the dispersion becomes unstable.
Alternatively, the net charge that resides on the particles to ensure their stabilization may be provided by charged molecules adsorbed onto their surfaces. A familiar example is provided by the same polystyrene latex particles (without the COOH groups), but in this case coated by molecules of an ionic surfactant, such as sodium dodecyl sulfate (SDS). The hydrophobic hydrocarbon “tails” of the SDS monomers are attracted to the surface and/or interior of the similarly hydrophobic polymeric particles, while the hydrophilic head groups are favorably disposed to lie on the surface, allowing maximal exposure to the surrounding water molecules. The majority of the Na+ ions are free to diffuse in the aqueous phase, leaving a net negative charge on the surfaces of the particles, by virtue of the SO3− groups belonging to the adsorbed SDS monomers.
As a consequence of the net charge residing on the surface of a given particle, there exists an electrical potential, ψ0, on its surface, with the zero value defined at an infinite distance from the particle. The value of the electrical potential, ψ (x), at a distance x from the surface of the particle decreases monotonically with increasing x, as shown schematically in FIG. 1. The distance parameter x is the normalized interparticle separation, defined as x=(r−2a)/2a, where r is the distance between the centers of the two particles (assumed spherical), and a is the radius of each particle (assumed uniform in size). The region immediately surrounding the surface of each particle, commonly referred to as the “Stern Layer,” contains a relatively high concentration of ions having a charge opposite in sign to that of the particle surface, attracted to the latter. The concentration of these oppositely-charged ions decreases progressively with increasing distance from the particle surface. Conversely, the concentration of charged ions having the same sign as that of the macro-particle increases with increasing distance from the latter. The diffuse region that extending beyond the Stern Layer, containing both positively and negatively charged mobile ions, free to diffuse between the macro-particles, is referred to as the Gouy-Chapman Layer. The behavior of ψ (x) vs x shown in FIG. 1 is not surprising, given the familiar 1/x behavior for an isolated charge, described by Coulomb's law. However, the potential typically falls significantly faster than 1/x with increasing x, due to the presence of electrolyte (i.e., salt ions) in the aqueous phase. The mobile charged ions serve to partially “screen,” or neutralize, the electrostatic field existing at a distance from the charged “macro” particle. The larger the concentration of added ions of opposite charge, the greater the screening of the electrical potential at any given distance from the particle.
Conversely, mobile ions having a charge of the same sign as that on the particle will be repelled by the surface, resulting in a relative deficiency of these ions close to the surface. The concentration of charged ions of opposite sign decreases, and the concentration of those of the same sign increases, with increasing distance from the particle, such that they approach the same average concentration far from the particle, where the value of ψ effectively falls to zero. The higher the concentration of added electrolyte(s), the faster the decay in ψ (x) with increasing distance, x. The distance at which the potential falls to 1/e times its value on the surface, ψ0, is often expressed as 1/κ, where κ is the “inverse screening length,” obtained from the well-known Debye-Hückel formula. The value of κ increases with increasing electrolyte concentration, depending on the square root of the latter (assuming monovalent ions). An increase in electrolyte concentration results in shrinkage (i.e. reduced “thickness,” 1/κ) of the electrical double layer.
The electrical potential extending outward from a given charged particle interacts with the charge carried by a neighboring particle, resulting in a repulsive force between the two particles. Of course, there is a corresponding electrical potential that extends out from the second particle. Therefore, the repulsive electrostatic force that exists between the two particles is often described as arising from the “intersection” of the two electrical double layers. The extent to which the two particles (or, in effect, the two electrical double layers) repel each other is accounted for analytically by the interparticle repulsive potential energy, VR, as described in DLVO Theory. The behavior of VR as a function of the separation, x, between the surfaces of two charged particles is shown schematically (curve “VR”) in FIG. 2.
The second fundamental influence on the stability of charged particles suspended in an aqueous medium is the London-van der Waals attractive force, originating from the interaction of electrical dipole moments in two neighboring particles. The dipole moment in one particle is induced by a dipole moment momentarily created in the other due to random fluctuations in local charge density. The strength of the interaction is characterized by the Hamaker coefficient, which depends on the composition of the particles. The London-van der Waals force is effective over only relatively short distances, falling off with particle separation, x, much faster than 1/x. That is, its decay with increasing distance is much faster than the corresponding decay exhibited by the interparticle electrostatic repulsive force in the absence of significant electrolyte concentration, owing to the longer-range nature of the latter force. The behavior of the resulting attractive interparticle potential energy, VA, as a function of particle separation is also shown schematically (curve “VA”) in FIG. 2.
The net potential energy, VTOT, that exists between two charged particles in aqueous suspension is therefore obtained by adding together the repulsive and attractive interparticle potential energies: VTOT=VR+VA. This result is also shown schematically in FIG. 2. For very small values of x—i.e. when the two particles are effectively touching—the value of VTOT is hugely negative, owing to the complete domination of the negative (attractive) London-van der Waals potential energy over the positive (repulsive) electrostatic energy at small separations. As the separation increases, the net potential energy rises steeply due to the diminishing influence of the short-range attractive force. Over these very small separation distances, the particles are, in effect, irreversibly agglomerated—i.e. they are trapped in the deep energy “well,” representing the lowest-energy state for the 2-particle system. As the separation distance increases further, the net potential energy eventually reaches a maximum value, VMAX, which is significantly positive provided the particles are adequately charged and the concentration of screening electrolyte is sufficiently low. Finally, VTOT rolls over and decreases with further increases in particle separation, x (i.e. assuming that the influence of VR is great enough to avoid the occurrence of a secondary minimum in the plot of VTOT vs x), eventually falling effectively to zero at large separations. The detailed behavior, or shape, of VTOT vs x depends on the magnitudes and shapes of the two competing potential energy functions, VR and VA, that combine to form VTOT.
The shape and magnitude of the net interparticle potential energy curve, VTOT vs x, shown schematically in FIG. 2, determines whether a particular charge-stabilized dispersion or emulsion will, in fact, remain stable, resisting agglomeration or coalescence over a long period of time. The peak in the VTOT vs x plot, having magnitude VMAX, constitutes an interparticle energy “barrier” that protects two charged particles from approaching each other too closely (by Brownian motion, or diffusion), lest they be so strongly attracted to each other that they fall irreversibly into the deep energy well created by the strong, short-range London-van der Waals attractive force. As the particles diffuse through the aqueous phase of the suspension, they possess an average kinetic energy on the order of kT, where k is Boltzmann's constant and T the absolute temperature. The greater the height of the interparticle potential energy barrier, VMAX, the more likely they will be repelled, failing to approach each other closely enough to allow irreversible agglomeration to occur. The higher the value of VMAX compared to kT, the larger the repulsive zone of “exclusion” surrounding each particle and the more infinitesimal the probability of penetration through this zone with each random attempt to do so.
It is instructive to review estimates of the influence of the interparticle potential energy barrier height on the “half-life” of a simplified “emulsion,” assuming droplets of a single size, 1 μm, with an oil/water ratio of unity. These estimates were calculated by Friberg et al (“Theory of Emulsions,” p. 63) in: Pharmaceutical Dosage Forms: Disperse Systems, H. Lieberman et al (eds.), Vol. 1, Marcel Dekker, New York (1988). The resulting droplet concentration is approximately 1011 per ml for the simplified emulsion. The half-life, t1/2, is defined as the time needed for half of the original number of droplets in the emulsion to agglomerate, or coalesce. The rate of agglomeration, or flocculation, is proportional to the square of the number of particles per unit volume, to the radius of the particles (assumed spherical) and to their diffusion coefficient. In the absence of charge stabilization, when the particles are free to collide with each other due to random diffusion, resulting in irreversible agglomeration, the half-life is less than one second. The presence of an interparticle potential energy barrier serves to slow down the rate of agglomeration, thereby increasing the half-life. In the case VMAX=5 kT (T=25° C.), t1/2 increases to only 38.2 sec. A doubling of the interparticle interparticle barrier height, to VMAX=10 kT, causes t1/2 to increase significantly, but only to 1.55 hr—clearly inadequate for a final product that typically must have a shelf life of many weeks or months, if not years. Another doubling of the interparticle barrier height, to VMAX=20 kT, results in a half-life of 3.91 years, approximating what is needed for many products of commercial significance. (Interestingly, an interparticle barrier height of VMAX=50 kT implies a half-life of 4.17×1013 years, surely representing a “stable” emulsion, where, for example, two days are required for the first pair of droplets (out of 1011 per ml) to coalesce.)
In principle, from DLVO theory one can estimate the height of the interparticle potential energy barrier (in units of kT) for a given charge-stabilized dispersion or emulsion, based on an independent measurement of the amount of charge residing on the particles, together with their size and the electrolyte concentration. Hence, in principle one should be able to estimate reliably the half-life of the dispersion or emulsion and achieve the desired stability by “fine-tuning” the chemical composition of the product. However, the same authors who provide the estimates of t1/2 vs barrier height summarized above are clearly pessimistic regarding the usefulness in practice of such theoretical approaches. Friberg et al (op cit, p. 66) write: “Calculations of the relative height of the barrier with electrolyte content are interesting from a scientific point of view, but of limited value in daily formulation efforts.” They (op cit, p. 70) also draw attention to the central problem associated with assessing emulsion stability—the need for large elapsed times to establish reliable conclusions. “The dilemma for the formulator of an emulsion lies in the fact that the success of a preparation can be judged only after a long time. If a shelf-life of one year is needed, it is in principle necessary to wait one year to find out whether a large number of samples still are intact.” This major shortcoming is also emphasized by Breuer (“Cosmetic Emulsions,” p. 420), in: Encyclopedia of Emulsion Technology, P. Becher (ed.), Vol. 2, Marcel Dekker, New York (1985). “Predicting long-term stability from accelerated laboratory tests still remains an elusive goal. In spite of its great commercial importance, only a small amount of fundamental research is being carried out on the problem. One of the reasons for this relatively low interest is, no doubt, due to the very long time periods that are required (e.g., 2 years of storage) for validating the results of any new predictive technique.”
The lack of fundamental understanding of the variables that influence the onset of coalescence (i.e. instability) in emulsions is underscored by Walstra (“Emulsion Stability,” p. 56), in: Encyclopedia of Emulsion Technology, P. Becher (ed.), Vol. 4, Marcel Dekker, New York (1996). “It may be clear from the discussion in this chapter that even for a relatively simple colloid like a (macroscopic) emulsion, much about its stability is insufficiently known. There are uncertainties about the colloidal interaction energy and its effect on aggregation rate. Coalescence especially needs further research, where the different variables should be studied separately. Also, partial coalescence would need more study, although the main variables have been identified and their importance has been established. Prediction of combined instabilities, which will often be encountered in practice, is far more difficult, although it has been tried in some cases.” Walstra (op cit. p. 119) goes on to speak to the uncertainty that exists regarding even the significance of coalescence in emulsions. “It may be concluded that the quantitative importance of coalescence is unknown. It is probably quite variable, and may be an important cause of the differences in droplet size found with different surfactants. Since coalescence probably takes place with freshly formed drops, this notion can be reconciled with the apparent existence of a critical drop size (larger ones are broken up) depending mainly on hydrodynamic conditions and much less on the dispersed phase.”
The deficiency in knowledge concerning the basic interaction phenomena in “complex” (i.e. multi-component) emulsions, such as milk protein systems, is acknowledged by Euston et al (p. 940), in: J. Food Sci, Vol. 65, pp. 934–940 (2000). “This work highlights two important points. First, the use of very simple systems may not be entirely appropriate when trying to predict the emulsifying characteristics of milk proteins in food systems.” “Second, although the interaction of milk proteins can have a large effect on the emulsifying properties of milk protein, there are gaps in our knowledge of which interactions are important, how they occur and how they can be exploited. There is a need for a systematic study of this area to allow us to use this information to better select ingredients for a particular application.”
The importance of emulsion stability, along with the apparent futility of accelerated stability testing, are underscored by Weiner (Introduction,” p. 9), in: Pharmaceutical Dosage Forms: Disperse Systems, H. Lieberman et al (eds.), Vol. 1, Marcel Dekker, New York (1988). “Coalescence is intolerable to all groups concerned and is a function of the strength of the emulsifier film at the droplet interface, that is, the interfacial free energy barrier. The factors affecting coalescence and the factors affecting creaming are very different, and accelerated stability testing for coalescence is, at best, difficult and tricky.” He continues: “With respect to predictive testing for coalescence of an emulsion or nonreversible settling of a suspension, there is little evidence that pushing the system far beyond what it will encounter in the marketplace yields any reliable information useful for shelf-life predictions. Moreover, overstressing the system creates the risk of throwing away formulations that would be perfectly acceptable under realistic conditions.”
Friberg et al (op cit, pp. 70–71) conclude their pessimistic assessment with the admission that faster, reliable testing methods do not exist. “This problem would be avoided if a reliable method for accelerated testing were found; that is, if a method were available that made it possible to judge the long-term behavior from short-term changes. Unfortunately, a general method of this kind is not available. There are methods in use that accelerate the destabilization process for emulsions of specific kinds, and these are useful within their realm of application. On the other hand, the important fact that these methods may give completely erroneous results when applied outside their established realm cannot be overemphasized.”
From the references cited above, it is clear that the central, unresolved problem in this field has been the prohibitively long time needed to establish whether, and to what extent, a given emulsion or dispersion is stable. Therefore, efforts have been made to accelerate the process of stability testing. Past attempts have, for the most part, centered around three means of accelerating the onset of instability of potentially problematic dispersions: 1) increasing (often substantially) the temperature; 2) inducing strong shear forces by mechanical means; and 3) speeding up the rate of sedimentation or flotation of large particles/droplets, by centrifuging the sample. Notwithstanding the usefulness of these various methods, for the most part they fail to yield unambiguous and/or consistently reliable quantitative information regarding the extent to which a given emulsion or dispersion is stable, or how it compares in quality to benchmark products of known performance.
Regarding the rationale for “thermal stress,” DLVO theory teaches that the stability of a dispersion or emulsion worsens with a decrease in the interparticle potential energy barrier height, VMAX, relative to a fixed value of kT. Hence, an increase in temperature for a fixed value of VMAX (i.e. an otherwise constant dispersion) should produce the same result—diminished stability. The extent to which the dispersion reveals symptoms of instability (e.g. the onset of phase separation in an oil-in-water emulsion) is therefore correlated with increases in temperature. The effect of temperature on the rates of chemical reactions (i.e. the Arrhenius equation), and hence on the stability of dispersions, notably drug formulations, is reviewed by Newton (“The Role of Temperature in the Life of a Pharmaceutical Preparation”) in: Pharmacopeial Forum, Vol. 25, #1, pp. 7655–7661 (January–February 1999), with corrections in Vol. 25, #4, p. 8627 (July–August 1999).
The use of elevated temperature to simulate the effects of aging on an emulsion-based reagent for performing assays of lipase is described by Kwan et al, in U.S. Pat. No. 5,378,609 (1995). The optical absorbance of a stable emulsion formulation indicated essentially unchanged composition and activity following thermal stressing at 57° C. for 12 days, equivalent to normal storage at 4° C. for four years. This behavior was in sharp contrast to what was observed for an unstable control sample, which began to degrade within seven days of storage at 4° C. and which showed immediate increased absorbance at 57° C., indicating the onset of precipitation.
Faure et al, in U.S. Pat. No. 6,347,884 B1 (2002), describe a method for determining the temperature stability of water-in-hydrocarbon emulsions with respect to phase separation, including crystallization of paraffins in gas oils at certain temperatures. The onset of phase separation is detected by monitoring the weight variations of a gravimetric sensor that is partially immersed in the water-oil mixture, used to enhance combustion efficiency.
Garver et al, in U.S. Pat. No. 6,263,725 B1 (2001), describe the use of an on-line sensor based on UV-visible light absorption and/or scattering to detect and identify colloidal substances, such as pitch or wood resin in pulp or paper process water. The difference or ratio of light attenuation or scattering measurements of a colloidal mixture performed at two or more temperatures provides a measure of the stability of the dispersion with respect to temperature. The observed differences in thermal stability for different samples provide a means for distinguishing different components of colloid-based fluids.
The method of raising the temperature to assess emulsion stability was also utilized by Yoon et al (“Interfacial properties as stability predictors of lecithin-stabilized perfluorocarbon emulsions”), in: Pharm Dev Tech, Vol 1, pp. 333–41 (1996). The oil-in-water emulsion used Pluronic F68 (a nonionic surfactant) to help stabilize the oil-droplet phase (by the mechanism of steric hindrance) and egg lecithin to provide additional charge-mediated stabilization. Thermal kinetic accelerated stability testing was conducted at 5, 20, 37 and 60° C., using multiple emulsion formulations and several means of particle size analysis to detect changes in the PSD of the droplets over a period of two months. In general, the number of days required to achieve the maximum measured mean diameter, “DMAX,” decreased with increasing temperature, as expected. However, there was a significant variation observed among the various emulsion formulations for some of the temperature values employed. The “Days to DMAX” period varied from 3 to 14 days at 60° C.; the period was uniformly 14 days at 37° C.; the period varied from 14 to 21 days at 20° C.; and the period was mostly 21 days at 5° C. The addition of cholesterol (i.e. added charged coating of the oil droplets) was found to improve the stability of the emulsion to thermal stress—i.e. it lengthened the period of time before noticeable changes in the PSD were detected. The significant variability observed in “Days to DMAX” for most of the temperatures employed only underscores the limitation associated with the use of temperature stress as a means of quantitatively evaluating the stability of typical multi-component emulsions.
Vadas (“Stability of Pharmaceutical Products,” p. 641) highlights the pitfalls associated with the use of elevated temperatures to accelerate stability assessment, in: Remington: The Science and Practice of Pharmacy, Gennaro (ed.), Vol. 1, Mack Publishing, Easton, Pa. (1995). “Two simple tests are used to screen emulsion formulations. First, the stability of an emulsion can be determined by heating it to 50–70° C., and its gross physical stability observed visually or checked by turbidimetric measurements. Usually the emulsion that is most stable to heat is most stable at room temperature. However, this may not be true because an emulsion at 60° C. may not be the same at room temperature. Second, the stability of the emulsion can be estimated by the “coalescence time” test. Although this is a rough quantitative test, it is useful for detecting gross differences in emulsion stability at room temperature.”
The first point made above by Vadas is that this seemingly straightforward approach of increasing substantially the temperature of an emulsion may, in practice, be seriously flawed. The typical increases in temperature that may be required to assess the stability of the emulsion using the principles of DLVO theory may be large enough to cause the emulsion itself to change. Depending on the complexity of its phase diagram, these increases in temperature may achieve the highly undesirable result of “converting” the emulsion or dispersion into a significantly “different” system, physically speaking, including having a significantly different PSD. This possible behavior is in sharp contrast to the desired goal of using the variable of temperature as a means only of perturbing the effectiveness of the net repulsive interaction between neighboring charged particles or droplets, while the character of the emulsion or dispersion is presumed to remain essentially unchanged.
The second approach that has been utilized for assessing emulsion stability is application of mechanical stress—e.g. “shaking” it in one form or another—in order to subject the dispersion or suspension to shear forces. The presumption is that application of moderate-to-strong shear forces will cause less stable emulsions to exhibit coalescence faster than more stable ones. For example, Degouy et al, in U.S. Pat. No. 5,257,528 (1993), describe a device for studying, on an accelerated basis the aging of a fluid circulating in a closed circuit—e.g., for testing the stability of muds used in oil drilling. The fluid is subjected to predetermined, elevated temperature and pressure and is circulated at a much faster rate than normally utilized, thus resulting in much higher shear forces. The fluid thereby undergoes accelerated aging, requiring only a relatively short time to reveal its Theological properties and, ultimately, its stability.
However, in practice, once again, this seemingly “straightforward” approach involving the application of mechanical stress for accelerated stability testing has proven to be fraught with uncertainties and difficulties. The limitations inherent in this approach can be appreciated by reviewing the communications between two pharmaceutical manufacturers of intravenous emulsions, in the form of Letters to the Editor (“Pharmaceutical and antimicrobial differences between propofol emulsion products”) published recently in Am J Health-Syst Pharm, Vol. 57, pp. 1174–76, 1176–77 (2000).
In the first letter, Redhead et al summarize the results obtained from accelerated physical stability testing (vigorous shaking) of their brand-name product, a phospholipid-stabilized 10% soybean oil-in-water emulsion (pH 6–9) containing 1% propofol, used for intravenous anesthesia and sedation purposes. A “wrist-action” shaker operating at 270 shakes/min for up to 16 hours was used to provide prolonged mechanical stress to the emulsion. Several particle size analysis techniques were used to assess changes in the PSD as a function of the duration of shaking. Results were compared with those obtained for a generic “equivalent” product (pH 4.5–6.4). Within two hours of shaking, the generic product exhibited large changes in the volume percentage of large-diameter (>2 μm) oil droplets, compared to no measurable changes after 16 hours for the brand-name product. The relatively poor stability of the generic product was attributed to its low pH, expected to reduce the negative surface charge (and corresponding zeta potential) of the phospholipid-stabilized oil droplets.
In their response to the Letter by Redhead et al, Mirejovsky et al dispute the validity of shake tests for providing a reliable measure of emulsion stability. They cite inconsistencies in the methods of mechanical agitation (i.e. oscillatory movement utilized in earlier studies referenced by Redhead et al, versus up-and-down movement associated with the wrist-action shaker actually used). They further cite the earlier work of Hansrani et al, in J. Parenter. Sci. Technol., Vol 37, pp. 145–150 (1983), acknowledging that oscillatory movement may cause emulsions to separate, but that if a formulation withstands sterilization, it will likely resist oscillation-induced disruption. Mirejovsky et al conclude: “These examples illustrate that a correlation between excessive shaking and the stability of an emulsion in real situations has never been established.”
Notwithstanding the potential difficulties, cited above, associated with the application of mechanical stress for accelerating the assessment of dispersion and emulsion stability, methods and equipment have been developed for implementing such testing procedures. An apparatus and method for evaluating the phase change of an emulsion is described by Date et al, in U.S. Pat. No. 5,319,958 (1994). A portion of the emulsion is applied to a sliding surface, and another surface is pushed against the latter during testing. A sensor means measures the force imparted to the pushing surface in the sliding direction. Changes in the measured force permit evaluation of the phase change of the emulsion. A method and apparatus for characterizing the dynamic stability (i.e. during flow) of emulsions is described by Joseph et al, in U.S. Pat. No. 5,987,969 (1999). An emulsion is fed through a gap defined between a stationary surface and a moving surface within a test vessel. The dynamic stability of the emulsion can thereby be characterized, based upon cycles of flow through the test vessel.
Lamar, III et al, in U.S. Pat. No. 3,950,547 (1976), describes methods for determining the stability of dietary emulsions following their preparation. The easiest, least sophisticated of these methods consists simply of “watching and waiting.” The percent volumes of separated fat, water-rich phase and oil-rich phase that have formed at various intervals of elapsed time, from one-half hour to 40 hours later are determined visually using a transparent, graduated vessel. Other methods involve the application of both thermal and mechanical stress to the emulsion. Thermal stability is determined by heating samples at 71° C. for two hours, followed by visual evaluation for signs of phase separation. Shear stability is determined by subjecting samples to high-speed (3200 r.p.m.) agitation in a Waring blender, again followed by visual inspection.
Centrifugation was listed earlier as a third means of testing emulsion stability on an accelerated basis. Vadas (op cit, p. 641) reviews the usefulness of centrifugation for assessing emulsion stability, where the droplets are assumed to have a lower density than that of the surrounding water phase. “The ultracentrifuge also is used to determine emulsion stability. When the amount of separated oil is plotted against the time of centrifugation, a plateau curve is obtained. A linear graph results when the oil flotation (creaming) rate is plotted vs the square of the number of centrifuge revolutions per minute. The flotation rate is represented by the slope of the line resulting when the log distance of emulsion-water boundary from the rotor center is plotted against time for each revolution per minute.” The progression of the emulsion-water boundary with elapsed time is typically followed by measuring the radial distribution of turbidity as a function of time, for various rotational speeds. The use of a centrifuge serves to decrease greatly the time needed to achieve significant oil flotation (creaming), compared to the long times required using a simple (gravity) sedimentation/flotation device, also based on turbidity. However, the centrifugal approach has at least one serious potential shortcoming. The act of progressive sedimentation or flotation of the suspended droplets causes large changes in the local concentration of these droplets. That is, a large oil-concentration gradient develops over time in the emulsion. Depending on the detailed nature of the phase diagram, it is entirely possible, if not probable, that the emulsion will be significantly changed in character, as opposed to being gently perturbed. This potential problem mirrors that which can occur due to excessive elevation of temperature, alluded to above by Vadas (op cit, p. 641).
Finally, Kanicky et al. (“Surface Chemistry in the Petroleum Industry,” p. 257) demonstrates the ongoing gaps in knowledge in emulsion technology, in: Handbook of Applied Surface and Colloid Chemistry, Holberg (ed.), Vol. 1, John Wiley and Sons, Ltd, West Sussex, UK (2002). Kanicky et al (op cit, p. 257) write: “It is clearly evident that emulsions are very complicated systems. Progress has been made on theoretical studies attempting to clarify the complexities of these systems. However, the majority of predictions of the type and stability of emulsions derives more from empirical observation than from theory. Emulsion formulation is still considered to be an art rather than a scientific method in many circles of industry.” Recent assessments by investigators, working in different fields of emulsion technology, draw attention to the shortcomings that exist in dispersion science. For example, Euston et al (op cit, 2000), Mirejovsky et al (op cit, 2000) and Kanicky et al (op cit, 2002) confirm that relatively little progress has been made in applying theoretical models to observed emulsion stability. These models include those discussed by Breuer (op cit, 1985), Friberg et al (op cit, 1988) and Walstra (op cit, 1996), applied to actual emulsions.